1. Field of the Invention
This invention relates to printing apparatus and, more particularly, to magnetic means for controlling the thickness of a toner layer applied to a transfer surface prior to printing.
2. Description of the Prior Art
Printing methods which employ an electrical field to move particulate printing material, toner, to a recipient sheet forming an image thereon are known in the prior art. One such printing process employs a printing sheet and a transfer surface or belt from which the toner is conveyed to a print head which establishes an electrical field for transferring the toner material to the recipient sheet or paper. The imprinted paper passes over a hot plate of sufficient temperature and length to fuse the paper and toner. A method of supplying toner to the recipient sheet that has been considered envisions using a permanently toned ribbon which would be used only once and then discarded. As this scheme is generally not practical, a continuous printer having a retoneable transfer surface or belt in contact with the toner, preferably a toner bead, for coating the belt has been considered. The rotating toner bead is comprised of fine magnetic particles and is formed by magnetic field gradients from a plurality of magnets employed above and below the bead and on opposite sides of the belt.
In general, the rotating magnetic bead scheme works well at low transfer belt velocities, that is, on the order of 20 inches per second, and when used with a rough surface belt. Present printing requirements, however, indicate that printing speeds on the order of 50-60 inches a second or more are desirous. However, as the rotating magnetic bead tends to fly apart at belt velocities on the order of 30 inches per second, a 60 inch per second belt velocity would require a bead four times larger in diameter and magnets eight times more massive (assuming the same magnetic material) than those originally used in the 20 inch per second system. Furthermore, at these faster speeds uniform coating and thickness control of the toner layer on the transfer surface is difficult.
Thickness control of the toner layer, as well as uniform coating of the transport surface, moreover, is required for good, dense printing, especially at the high speeds, 50-60 inches per second, now being considered. That is, at the present speeds of about 20 inches per second, the belt transports the toner layer thereon past the print head at approximately the same speed as the recipient paper is moved past the print head, because the transport surface is coated with a toner layer of limited area density. Furthermore, the present limited area density is approximately equal to the area density of toner required to be transferred onto the paper for high quality, dense printing. In other words, in the present systems substantially total transfer of the toner layer from the belt to the paper is necessary to achieve the required printing quality at 20 inches per second. If the density of the toner layer carried by the belt could be controlled, e.g., substantially increased, that is, if the toner layer thickness could be controlled, the transport surface could be run at a substantially reduced speed from that of the paper, thereby reducing, among other factors, the possibility of toner spillage.
Accordingly, there is a need for providing means for providing a dense toner layer and for controlling the thickness of the toner layer for relatively fast transport surface speeds without sacrificing good printing characteristics.